1. Field of the Invention
The present invention relates to a suction pad for holding a substrate and a substrate treatment apparatus having the same, and in particular, to a suction pad for holding the peripheral part of the rear face of the substrate using vacuum suction so that the rear face of the substrate is not treated and a substrate treatment apparatus having the same.
2. Description of the Related Art
As a substrate treatment apparatus that treats only the front face side of the substrate so that the rear face of the substrate is not treated while the substrate is immersed in a solution, an anodizing apparatus is disclosed, for example, in Japanese Patent No. 3,376,258.
FIG. 5 is a schematic view of the anodizing apparatus illustrated in Japanese Patent No. 3,376,258. The anodizing apparatus has an anodization tank 108 for storing a HF solution 107, a holder 102 integrated with the anodization tank 108, and electrodes 106a and 106b disposed to the anodization tank 108 for anodizing a silicon substrate 101 by applying a voltage to the silicon substrate 101. An opening 103 is provided to the holder 102, a groove is provided to the holder 102 along the opening 103, and a suction pad 104 is fitted in the groove.
The suction pad 104 has a holding portion which attaches to a part of the rear face of the silicon substrate 101, a suction groove 104a whose cross-sectional shape is concave or U shaped, and a suction opening 105 for depressurizing the space in the suction groove to cause the substrate to attach.
In a substrate treatment using the anodizing apparatus, first, the silicon substrate 101 is immersed in a HF solution 107 while being subjected to vacuum suction with respect to the suction pad 104 via a pump (not shown) connected to the suction opening 105 through a line. To the silicon substrate 101 immersed in the HF solution 107, a current is flown by applying a voltage to the electrodes 106a and 106b. After that, upon finishing of the treatment of the silicon substrate 101 by the application of the voltage, the HF solution 107 is drained from the anodization tank 108, and then the silicon substrate 101 is exposed to the atmosphere. In order to allow the silicon substrate 101 to be taken out of the anodization tank 108, an inert gas such as nitrogen is poured in the line so as to break the vacuum atmosphere that caused the silicon substrate 101 to attach to the suction pad 104 via the suction opening 105. By this procedure, the anodization treatment of the silicon substrate 101 is completed.
Since the silicon pad 104 has the above-described shape and relationship with respect to the silicon substrate 101, even when the silicon substrate 101 is immersed in the HF solution 107, the HF solution 107 will be brought into contact with only the front face of the silicon substrate 101. Accordingly, this prevents the HF solution 107 from being brought into contact with the rear face of the silicon substrate 101. By applying a voltage to the electrodes 106a and 106b to flow a current in the silicon substrate 101, a porous layer is formed only on the front face of the silicon substrate 101, and is not formed on the rear face of the silicon substrate 101, because the HF solution 107 is not brought into contact with the rear face of the silicon substrate 101.
As illustrated in FIGS. 6A and 6B, during such a substrate treatment, in some cases, the HF solution 107 erroneously enters the line through the gap between the silicon substrate 101 and the suction pad 104. When vacuum is broken, the HF solution 107 that has entered the line will flow back in the vacuum suction line being pushed back by the inert gas. In this case, the flowing-back solution may further pass through the gap between the suction pad 104 and the silicon substrate 101 or the suction pad 104 and the holder 102 and spurts out near the end of the substrate. In particular, when the spurting part of the HF solution locates upper than the silicon substrate 101, the spurting HF solution 107 will come around to the front face of the silicon substrate 101 and then pass along and drop from the front face of the silicon substrate. Since many foreign materials comprised primarily of reaction by-products are contained in the solution which entered the vacuum suction line, if the solution passes along the face of the silicon substrate 101, the foreign materials may attach to and remain on the passage, or watermarks may be formed on the passage since liquid droplets remain and get dried on the front face of the substrate.
Moreover, in such a production method of a SOI wafer disclosed in U.S. Pat. No. 5,371,037, a SOI wafer is produced using a porous silicon layer formed by a method described in U.S. Pat. No. 6,517,697. Foreign materials as mentioned above may affect the quality of SOI wafers. For example, once a solution drawn into the vacuum suction line flows back and attaches to the produced porous silicon, foreign materials remain on the face of the porous silicon. As a result, since an epitaxial silicon layer is not formed on the position to which the foreign materials are attached, resulting in defect of a SOI wafer where a SOI layer is lacked. Such lack of the SOI layer, when device chips are produced, may cause malfunction of a circuit.
In other words, in order to reduce the foreign materials and watermarks on the face of the porous silicon, it has been required to prevent the solution that flows back from the vacuum suction line as mentioned above from coming around to the substrate front face.
The present invention is completed in view of the above problems, and, in a substrate treatment method for treating only the substrate face using a liquid chemical, the present invention provides a suction pad capable of preventing undesired foreign materials from attaching to the substrate face when the substrate is taken out, and a substrate treatment apparatus having the same.